Abstract

Based on a straightforward physical model, a new lightning parameterization has been developed:
A two-plate capacitor represents the basic dipole charge structure of a thunderstorm, which
is charged by the generator current and discharged by lightning. In this approach, the
generator current as well as the discharge strength are parameterized using the graupel-mass field.
If these two quantities
are known, and if the charging and discharging are in equilibrium, then the flash rate is uniquely
determined. This approach remedies shortcomings of earlier theoretical approaches that relate the flash
rate e.g., to generator power. No distinction is made between intracloud and
cloud-to-ground discharges.
In order to test this approach, polarimetric radar
data were used, from which the graupel distribution in observed thunderstorms could be inferred. The lightning
activity was detected using the LINET network. The comparison between
theoretically-predicted and measured flash rates is encouraging: Over a wide range of flash rates,
the theoretical approach yields accurate results for isolated thunderstorms. Two existing
parameterizations, which only use the depth of the clouds as predictor, produce
substantially less accurate forecasts.
These two existing approaches, the one developed in this study, as well as a fourth one based on updraft velocity,
were implemented in the convection-resolving COSMO-DE numerical weather prediction model. With this model,
real-world convective scenarios were simulated. The output of the lightning scheme includes the location and time
of every simulated discharge.
Testing the performance of the
parameterizations with modeled convection is difficult as there is no one-to-one correspondence between
observed and modeled convective clouds. Where a comparison between modeled and observed flash rates of individual clouds
was possible, the results for individual cells were promising.
The comparison of the bulk lightning activity over an area comprising southern Germany and adjacent countries
suggests that none of the four parameterizations captures the overall lightning activity well. This is mainly
because COSMO-DE does not simulate the observed number of cells at the correct times.